Subtitle displays for layers which play video

Subtitles have been discussed a lot in some of the a11y discussions such as in XAUR. Ideally a video with a subtitle track played through layers should display the subtitle in a way which works well and makes sense.

Here is the example mark up for a video with a video track: https://developer.mozilla.org/en-US/docs/Web/Guide/Audio_and_video_delivery/Adding_captions_and_subtitles_to_HTML5_video#html_markup

It would be handy if the subtitles can be positioned by the UA outside of the layer boundary so a video being played off to the side can still be "listened to" using the subtitles.

It would also be handy if the subtitle layer could be used by elements with subtitle tracks.

If they can also be styled with the ::cue pseudo class that would also be very great.

https://developer.mozilla.org/en-US/docs/Web/Guide/Audio_and_video_delivery/Adding_captions_and_subtitles_to_HTML5_video#styling_the_displayed_subtitles

Start with the HTML video and audio subtitle-track markup and the linked MDN guidance on captions, positioning, and ::cue styling. The issue also references XAUR accessibility guidance. Done would require an agreed design for subtitle display, positioning outside layer boundaries, audio support, and styling, but no files or tests are identified.